The present invention relates generally to the field of data communications, and more specifically, to the field of synchronous, fixed boundary, variable data rate communication systems, such as code division multiple access (CDMA) North American digital cellular telephone and personal communication systems.
Synchronous communication systems which utilize fixed frame boundary data frames including data at variable rates are known in the art. One example of such a system is the CDMA North American digital cellular system, a well-known class of modulation using specialized codes to provide multiple communication channels in a designated segment of the electromagnetic spectrum. Thus, the definition of "synchronous" is understood to include all systems in which an attempt is made in at least one transmission direction to synchronize system timing (frame and bit timing are recoverable) between transmitting and receiving stations. The Telecommunications Industry Association (TIA) has standardized a CDMA implementation in the "Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System TIA/EIA/IS-95 Interim Standard" (IS-95) and the "Speech Service Option Standard for Wideband Spread Spectrum Digital Cellular System TIA/EIA/IS-96 Interim Standard" (IS-96). Sections 6-6.2.4 and 7-7.2.4 of IS-95 and the entire IS-96 are particularly relevant. In addition, updated versions of these standards, known as IS-95A and IS-96A, are also available. Of particular note in these updated versions are guidelines associated with a second rate set accommodating a higher speed vocoder.
Another example of a variable data rate communication system is the CDMA personal communication system described in the industry standard TIA proposal no. 3384, published as J-STD-008, entitled "Personal Station Base Station Compatibility Requirements for 1.8 to 2.0 GHz Code Division Multiple Access (CDMA) Personal Communication Systems". While other sections are also relevant to the present invention, sections 2.1.3.3-2.2.3 of J-STD-008 are particularly relevant. As would be understood by one reasonably skilled in the art of the present invention, the personal communication system (PCS) mobile and base stations of J-STD-008 are very similar to the mobile and base stations, respectively, of IS-95A except for the operating frequencies, thus, unless otherwise noted, the term "mobile stations" should be understood to refer to cellular mobile stations and personal communication stations.
In the conventional CDMA digital cellular and personal communication systems, variable data rates are utilized to reduce the data transmission rate during times of reduced speech activity. This data rate reduction results both in a reduction of interference with other users (thereby increasing capacity in the system) as well as in a reduction in average transmit power of the CDMA mobile station (thereby increasing battery life). On the transmitter end (transmitting base station or transmitting mobile station), a vocoder (voice or speech encoder/decoder) compares voice energy levels to adaptive thresholds based on background noise levels to determine an appropriate data rate for each frame of speech data, thereby suppressing background noise and providing good voice transmission in noisy environments. Using a code excited linear prediction (CELP) method, the vocoder receives pulse code modulated speech samples and reduces the number of bits required to represent speech by exploiting the intrinsic properties of speech signals to remove redundancy. Subsequently, the speech encoded data is convolutionally encoded for forward error correction before being interleaved and modulated for transmission.
Since the data rate may change at each frame boundary, the CDMA receiver must first determine the data rate of each frame of data. The process by which this is accomplished in the conventional CDMA digital cellular and personal communication systems is a source of wasted time and processing energy. According to the conventional systems, each data frame must be separately processed at each of the various possible data rates (including convolutional decoding) before a decision is made regarding which data rate was utilized on the transmitter end. Since this method is clearly inefficient, there is a need in the industry for a new method for determining the data rate of each frame of data in the CDMA digital cellular and personal communication systems, as well as other systems using fixed boundary frames with variable data rates.
One possible method of addressing this problem is the addition of a conventional header before each frame of data. Such a header could include the data rate of the corresponding frame to which it is attached. Unfortunately, such a header would also need error protection to reduce the likelihood of transmission errors. In view of the relatively small size of each frame of data, the additional bits required for an error protected header would certainly add substantial overhead and undesirable complexity to the system.
There is, therefore, a need in the industry for a system which addresses these and other related, and unrelated, problems.